Liquid compositions of perfluorinated ion exchange polymers are known for use in the manufacture and repair of ion exchange membranes, for membrane coatings containing conductive and non-conductive particles, and for many other uses. While such compositions are sometimes referred to as solutions, the compositions are generally recognized as being dispersions of polymer particles.
Liquid compositions are typically prepared by suspending in an appropriate aqueous or aqueous-alcoholic medium the perfluorinated ion exchange polymer. Methods useful for obtaining such liquid dispersions are notably taught in U.S. Pat. No. 4,433,082 (DUPONT DE NEMOURS) 21 Feb. 1984, GB 1286859 (DU PONT) 23 Aug. 1972, EP 1004615 A (AUSIMONT S.P.A.) 31 May 2000 and U.S. Pat. No. 6,150,426 (DUPONT DE NEMOURS) 21 Nov. 2000.
Nevertheless, the liquid compositions of the prior art possess limited film-forming capabilities. In particular, due to their high viscosities in the liquid state and their poor tixotropic and/or rheological behaviour, formation of thick coatings is not possible. In other words, critical film thickness above which film quality is unsatisfactory is limited, so that several repeated coating/casting steps are required for obtaining final target thicknesses in the membrane.
Pastes or electrode forming compositions comprising ionomers of different equivalent weight are also known in the art.
Thus, JP 10284087 (ASAHI CHEMICAL IND) 23 Oct. 1998 discloses a fuel cell wherein the catalyst layer is made from a paste comprising at least two different fluoroionomers having different EW, for improving water removal from electrodes. According to this document, the two perfluoroionomers shall be selected to have a difference in equivalent weight (EW, hereinafter) of at most 800, and at least 20.
US 2004053111 (ASAHI CHEMICAL IND.) 18 Mar. 2004 discloses an electrode for solid polymer electrolyte fuel cell wherein electrocatalyst particles are covered with a first ionomer (so-called “primary presence state” (1)) and said “coated” particles are binded together by a second ionomer (so-called “secondary presence state” (2)), that is to say that the so-manufactured electrode comprises two ionomers having different EW. Equivalent weights of said ionomers are selected according to this document so as (1) possesses an EW of 500 to 1000; and (2) possesses an EW of 850 to 1500. The electrodes according to this document are prepared by first mixing the catalyst (e.g. Pt supported on C) with the perfluorocarbon sulfonic acid of type (1) in a H2O/solvent mixture; then the so-obtained paste is again mixed with the perfluorosulfonic acid of type (2) in the same medium.
US 2005043487 (DUPONT DE NEMOURS) 24 Feb. 2005 discloses fluoropolymer compositions suitable for providing membranes for fuel cells by extrusion moulding, said composition comprising from 70 to 95 wt. % of a fluorinated ionomer; and from 30 to 5 wt. % of at least two melt processible fluoropolymers substantially free of ion-exchange groups, differing each other in melting point by at least 5° C. This document teaches that the fluorinated ionomers differ from each other by their equivalent weights. Thus, examples relate to mixtures of fluoroionomers (copolymers of TFE and perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) having an EW of, respectively, 1000 and 920, in combination with FEP and PFA used for manufacturing membranes by extrusion moulding.
US 2005186467 (ASAHI KASEI) 25 Aug. 2005 discloses an electrode for a polymer electrolyte fuel cell, wherein the catalyst layer is manufactured by application of a catalyst paste, said paste containing supported catalyst particles and an electrolytic composition in which electrolytes of different equivalent weight are dissolved or dispersed.
None of the pastes of the prior art could provide for good filmability properties in combination with appreciable behaviour in fuel cells operations.
There is thus a current shortfall for liquid dispersions suitable for forming films, said dispersions having outstanding filmability properties, and films produced therefrom having valuable behaviour in fuel cells operations, with virtually no failure and substantial reduction of hydrogen cross-over phenomena during operations.